1. Field of the Invention
This invention relates to nuclear reactor shutdown mechanisms and more particularly to a temperature responsive, self-actuated nuclear reactor shutdown control rod assembly.
2. Prior Art
State of the art nuclear reactor designs include emergency shutdown systems which incorporate the introduction of some form of neutron absorbing material into the reactor core.
One type of emergency shutdown system responds to the reactor coolant pressure changes resulting from loss of coolant flow due to coolant pump stoppage. An example of an emergency shutdown system which responds to pressure changes is U.S. Pat. No. 3,228,847 issued to T. F. Parkinson which suggests a reactor control system which includes a control assembly for controlling neutronic flux. The control assembly comprises an inner tube extending from a nonactive region of the reactor into the active region, and an outer tube surrounding the inner tube and spaced therefrom. The outer tube has a closed end and the inner tube has an open end adjacent and spaced from the closed end of the outer tube. Neutron absorbing particles are positioned between the inner and outer tubes for movement along the tube under the force of flow. The neutron absorbing particles are moved out of the active region of the reactor by fluid flow and fall back into the active region under the influence of gravity when the flow is shut off.
Another example of a shutdown system which responds to reactor coolant pressure changes is U.S. Pat. No. 3,347,747 issued to J. M. West which discloses a control organization and method for a nuclear reactor. The reactor is provided with a number of laterally spaced vertical passageways in the region of the core and distributed throughout the area thereof. The passageways include a lower portion which extends generally throughout the height of the core and an upper portion which extends above the core into the reactor vessel. Positioned within and confined in each passageway is a movable means which contains a poison and which is movable from a power position within the region of the core to an upper position in the passageway, where it is generally above the core. The poison-containing means is moved by gravity to its lower position and is moved from its lower to its upper position by means of a fluid which is directed upward in the passageway.
Other systems which rely upon pressure changes include, for example, U.S. Pat. No. 3,257,286 issued to J. W. Ryon, U.S. Pat. No. 4,076,583 issued to E. B. Ash, and U.S. Pat. No. 4,313,794 issued to D. K. Chung.
Use of systems such as the foregoing systems offer advantages over reliance solely on a control rod system. However, in all of these systems where coolant pressure is relied upon to cause the absorber elements to move into the core, any residual fluid flow, even though it may be below the minimum for safe operation of the reactor, acts to retard the fall of the absorber elements.
Furthermore, these pressure-activated systems will not react in the case of reactor overpower accidents because reactor coolant pressure does not drop during an overpower condition.
A second type of emergency power control system responds to reactor coolant temperature. In U.S. Pat. No. 4,227,967, issued to E. L. Zebrowski, an apparatus is disclosed which comprises a series of bimetallic elongate expansion members with high and low thermal expansion coefficients which expand to primary coolant temperature rises. The expansion members are disposed in alternate relationship and connected in series alternately at their top ends and at their bottom ends. The expansion members are connected at one end to the drive line and at the other end to absorber material. When the temperature rises the expansion members elongate putting the absorber member into the reactor core.
Since the Zebrowski device is temperature responsive it reacts to loss of coolant flow due to coolant pump stoppage and also to reactor overpower conditions. Another advantage of this device is that it does not rely on mechanical features to be activated.
However, the Zebrowski device responds in a linear fashion to temperature changes in the reactor. As a result, it will respond to temperature rises well within normal operating temperature limits. This results in unwarranted and undesirable power fluctuations. Additionally, in order to get meaningful elongation out of the expansion members a multitude of members is required. The requirement of a multitude of members might make use of this apparatus prohibitive in view of the limited space within the upper structure shroud of the reactor.